1. Field of the Invention
The present invention relates to a resonant switching power supply device and particularly to a resonant switching power supply device able to reduce an increase in switching frequency when a load is light.
2. Description of the Related Art
As a DC-DC converter, a resonant switching power supply device has been used. The resonant switching power supply device is equipped with: high-side and low-side switching elements that are connected in series between the cathode and anode of an input direct-current power source; and a resonant circuit that is connected in parallel to one of the switching elements and consists of a primary winding of a transformer and a current resonant capacitor. A resonant current flows through a primary-side inductance element of the transformer and the current resonant capacitor as the switching elements are turned on and off. At this time, the voltage obtained at a second winding of the transformer is rectified. A direct-current voltage is therefore obtained at the output side.
The resonant switching power supply device can adopt a control method called PFM (Pulse Frequency Modulation) control to control an output voltage by changing the switching frequency. When a higher frequency range than a resonant frequency is used as the switching frequency of the PFM control, the switching frequency is raised with the lower output voltage and the switching frequency is lowered with the higher output voltage. In this case, the output voltage is fed back and the switching frequency of the switching elements is automatically controlled so that an output direct-current voltage gets stabilized.
In the resonant switching power supply device, there is the following phenomenon: an oscillating voltage is superimposed on a secondary-side voltage due to the effects of a stray capacitance of the transformer and a leakage inductance when a load is light, an output smoothing capacitor is charged to the peak due to the oscillating voltage, and the output voltage rises as a result. In the resonant switching power supply device, the output voltage is fed back and the switching frequency is PFM-controlled so that the output voltage gets stabilized. The problem is that when the output voltage rises after the output smoothing capacitor is charged to the peak due to the oscillating voltage as described above, the switching frequency then rises in a way that reduces the increase of the output voltage.
To solve the above problem, for example, what is disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2009-100631 (Patent Document 1) is a technique of providing a capacitor in parallel to a current resonant reactor connected in series to the primary winding of the transformer to prevent the increase of the switching frequency when the load is light. The technique of Patent Document 1 is aimed at leading a charge current of the stray capacitance to the parallel-connected capacitor by bypassing the current resonant reactor to prevent unnecessary boosting energy from being stored in the current resonant reactor and keeping the switching frequency from rising when the load is light.